Originally Posted by '05Train
As I'm sure you're aware, the link you posted was of a final drive unit, not the lower 1st gear that the GS/A has.
Yep. It meets your criteria for "bolt on gearing change". As for the enduro transmission; it was an available option in '08-'09 but they didn't sell and were discontinued.
Honestly, I gotta wonder about the advantage of the lower first gear anyway. One of the moderators here, AceRph, has a hacked (sidecar rig) '08 GSA. He blew up the trans in it, and after searching for months, finally found a standard GS trans for a reasonable price. He said he can feel the slight difference, but it don't seem to be a problem and don't bother him at all.
The guy who built Roger's rig, Jay at DMC (Dauntless Motors in Seattle) has a hacked standard GS, and the only times he's wished for a lower first gear in many thousands of miles was loaded with camping gear, with his wife in the chair, and towing a tent trailer.
The combined weight of the chair plus tent trailer is more than the weight of either bike in question.
The lower first gear can certainly be an advantage under some circumstances, but almost nobody ever meets that criteria.